feat(mediatek): remove bl32 flag for mtk_bl

Currently MediaTek platform code does not support the bl32 image.
Remove bl32 support from Makefile to prevent the build failure when
NEED_BL32 build flag is enabled.

Change-Id: Id8d5663ea5c537390f8ff3ccb427a3a63266545e
diff --git a/plat/mediatek/build_helpers/mtk_build_helpers.mk b/plat/mediatek/build_helpers/mtk_build_helpers.mk
index 83a4dd2..ac2cbad 100644
--- a/plat/mediatek/build_helpers/mtk_build_helpers.mk
+++ b/plat/mediatek/build_helpers/mtk_build_helpers.mk
@@ -103,12 +103,8 @@
 MTK_OPTIONS := $(MTK_PLAT)/build_helpers/options.mk
 MTK_COND_EVAL := $(MTK_PLAT)/build_helpers/conditional_eval_options.mk
 
-# Indicate which BL should be built in command line
-ifeq (${NEED_BL32},yes)
-MTK_BL := bl32
-else
 MTK_BL := bl31
-endif
+
 # Include common, platform, board level config
 include $(MTK_COMMON_CFG)
 include $(MTK_PLAT_CFG)